Writer-director Joshua Marston’s “Complete Unknown” is a good illustration of a certain type of Sundance film — it’s neither more nuanced, literate or character-focused than an ordinary Hollywood studio film. Instead, it’s simply a really thin, weak imitation of a Hollywood studio film. Marston has one idea, and doesn’t know what to do with it. With the exception of the “reveal,” as they say at the script meetings, the whole movie feels like filler.

The film is a showcase for Rachel Weisz, who plays a biologist with an air of mystery who strikes up a friendship with the co-worker (Michael Chernus) of a New York City government consultant (Michael Shannon) who has a boring job drafting strategic advice for legislation. The latter is about to host a birthday party with his wife (Azita Ghanizada), a jewelry designer whom he is squabbling with about her proposed move to San Diego to take a course.

There’s a secret in here somewhere, but, fatally, it takes half the movie for Marston to tease it out. Bad choice: The secret is simply the “what if?” that a competent screenwriter would use as a launching pad for lots of other twists.

Instead, the movie floats along, never quite even establishing a genre for itself. In what ought to be a riveting scene, the Weisz and Shannon characters go out for a walk and meet a friendly old woman (Kathy Bates) who is walking her dog when she suffers an injury. Weisz and Shannon conspire to play a kind of con game in the process, but while it appears that the stakes are getting raised, Marston lacks ideas about how to build on what he’s got.

The scene trickles out without changing the overall direction of the story, just as the mystery woman at the center of the film never really comes into focus before, at the end, fading out. Whether you classify it as a psychological thriller, a character study or a reflection on our times, “Complete Unknown” is a failure: desultory, lackluster, mild and instantly forgettable.